Session 4 Flashcards
State the solubility of oxygen in body fluids
[*] The solubility coefficient of O2 = 0.01mmol/L / kPa at 37oC
[*] Therefore at a partial pressure of 13.3kPa and a temperature of 37oC plasma contains 0.13 mmol/L of dissolved oxygen (0.01 x 13.3)
[*] Oxygen is not sufficiently soluble in body fluids for adequate gas transport in simple solution. Even if all the oxygen could be extracted, cardiac output would have to be impossible high and a chemical reaction would be needed to transport more O2 per litre of blood
[*] At rest we need 12mmol of O2 per minute. The volume that would contain this amount is 92 litres.
Draw an oxygen-haemoglobin dissociation curve, label the axes correctly and indicate the normal values of (i) alveolar pO2 and (ii) capillary pO2 in a typical tissue
[*] We need to increase the amount of oxygen taken up by blood in the lungs whilst still ensuring substantial unloading in the tissues
[*] Haemoglobin reversibly loads and unloads oxygen over a very narrow range of pO2
[*] The interaction between Haemoglobin and oxygen – the reversibility of oxygen binding - is reflected in the Haemoglobin oxygen dissociation curve
[*] Normal alveolar pO2 is 13.3kPa
[*] Normal capillary pO2 in a typical tissue is ~5kPa
[*] Dissociation curves tell you how much oxygen will be bound or given up when blood is moved from one pO2 to another (by working out the difference in fractional saturations between the two pO2s and multiplying it by the amount bound at full saturation to tell you how much oxygen is taken or given up).
List the properties of the haemoglobin molecule which facilitate the transport of oxygen in the blood
[*] The normal concentration of Haemoglobin in blood is about 2.2mmol/L. Each Haemoglobin molecule binds 4 oxygen molecules when saturated.
[*] Hb reversibly binds to oxygen over a vary narrow range of ppO2.
[*] It is a tetrameric protein (2alpha,2beta subunits) containing 4 haem groups. It has a variable quaternary structure
[*] Hb can exist in two states – a low affinity T-state (tense) and a high affinity R-state (relaxed).
[*] When Haemoglobin is in the R state, it binds to oxygen very easily (high affinity). When haemoglobin is in the T state it does not bind oxygen very well.
[*] Transition between these two states gives Hb its sigmoidal binding curve, so Hb’s affinity to O2 increased as more O2 binds (cooperative binding).
- Hb gets tense when pO2 is low so it is hard to bind the first oxygen as most molecules are in the tense form. As oxygen binds to haemoglobin, Hb becomes more relaxed so binding the next oxygen is much easier as any more molecules relax. Binding facilitates further binding so the dissociation curve rapidly steepens as pO2 rises until saturation.
- Haemoglobin is saturated above 8.5kPa and virtually unsaturated below 1 kPa. It is half-saturated at 3.5-4 kPa so saturation changes greatly over narrow range of pO2 – and it is a highly reversible reaction.
Draw the effects on the haemoglobin oxygen dissociation curve of (i) a fall in pH and (ii) a rise in temperature
[*] Blood leaves the lungs carrying virtually all the oxygen it can. Increasing the pO2 in all or part of the lungs will not increase the content of oxygen in the blood.
[*] The unloading of oxygen in the tissues depends upon the fall in pO2 in the capillaries and changes in haemoglobin produced by the different conditions in the tissues.
[*] H+, increasing temperature and increased pCO2 decrease the affinity of Hb for O2 – Hb is more tense. At sites of low pH (high [H+]) and increased CO2 for example muscle tissue during exercise, more oxygen is required and will be released (the Bohr effect)
[*] The oxygen dissociation curve shifts to the RIGHT so at any pO2, Hb binds less oxygen
[*] In the tissues pH is lower so extra oxygen is given up.
[*] Maximum unloading: in tissues where pO2 can fall low, conditions are acidic and warm and about 70% of bound oxygen is given up. Over the whole body, ~27% of the oxygen in arterial blood is given up to the tissues. Much more is given up in exercise – drawing on the ‘oxygen reserve’ in our blood.
[*] When oxygen tension is low, 2,3 Diphosphoglycerate (2,3 DPG) accumulates in red blood cells. This also binds to haemoglobin and shifts the dissociation curve to the right in both tissues and lungs.
Estimate the rate of delivery of oxygen to the tissues at different capillary pO2s and pHs
[*] In the lungs, alveolar pO2 is 13.3 kPa so Hb is well saturated. Hb in normal blood is 2.2mmol/L so each molecules binds 4 oxygen so oxygen content is 8.8mmol/L.
[*] Tissue pO2 varies but typically 5kPa (can’t go much lower because you need a pressure gradient to drive O2 out of the capillary into the tissue)
[*] Hb is now about 65% saturated
- Change in binding = 100 -65%
- Oxygen given up is 8.8 x 0.35 mmol/L = ~3mmol/L
[*] Haemoglobin in venous blood still has over half its oxygen bound so tissues could remove more.
[*] If the pO2 in the capillaries of tissues falls, pH falls and temperature rises so that Hb will give up more oxygen. Therefore the saturation of Hb leaving the capillaries will be greatly reduced.
[*] If venous pO2 is known, a dissociation curve can be used to calculate the percentage of oxygen that has been given up to that tissue.
- The extent that capillary pO2 can fall without compromising diffusion of oxygen to cells depends mainly on capillary density. It can fall further in tissues such as the myocardium which have many capillaries. HOWEVER tissue pO2 must be high enough to drive oxygen out to cells so it cannot fall below 3kPa in most tissues
State the factors influencing the diffusion of gases across the alveolar membrane
[*] The rate at which gases exchange is determined by three factors:
- Area available for the exchange
- Resistance to diffusion (structure of the alveolar membrane, extracellular fluid barrier and capillary wall)
- Gradient of partial pressure
[*] See Session 2
[*] Normally diffusion rate is sufficiently fast to allow full gas exchange in ½ second – half the time the blood spends in the capillaries.
[*] Under normal conditions therefore, the blood leaving the alveolar capillaries has the same gaseous composition as alveolar air. Arterial gas tensions are therefore determined by alveolar gas composition, so respiration must be controlled to keep alveolar pCO2 at 5.3kPa and alveolar pO2 at 13.3kPa
[*] Oxygen diffuses more rapidly in the gas phase but CO2 much more rapidly in the liquid phase so that overall CO2 diffuses 21x as fast as oxygen.
Describe in outline how the transfer factor (‘diffusion capacity’) of the lungs may be determined
[*] Transfer factor may be calculated with carbon monoxide
[*] The diffusion capacity is the resistance to diffusion across the alveolar membrane.
[*] The Carbon Monoxide Transfer Factor measures the rate of transfer of CO from the alveoli to the blood in ml/min/kPa – the amount transferred will depend on how well gas diffusion takes place
[*] Inhaled CO is used because of its very high affinity for Hb. Since almost all the CO entering the blood binds to Hb, very little remains in plasma so we can assume plasma ppCO is zero. Therefore the concentration gradient between alveolar ppCO and capillary ppCO is maintained. As a result the amount of CO transferred from the alveoli to the blood is limited only by the diffusion capacity of the lung.
[*] The patient performs a full expiration followed by a rapid maximum inspiration of a gas mixture composed of air, a tiny fraction of CO (0.1%) and a fraction of an inert gas e.g. helium (14%) (used to make an estimate of total lung volume)
[*] Breath is held for 10 seconds
[*] The patient exhales and after discarding the first 750ml, the next litre is collected.
[*] Concentrations of CO and Helium are measured – the alveolar pCO and CO uptake are easily measured so the apparent diffusion resistance or “transfer factor” may be calculated. The helium in the mixture allows correction for the diluting effects of air in the lungs (residual volume) when the measurement begins.
What diseases could lead to a low carbon monoxide transfer factor?
[*] Diseases affecting diffusion (Low Carbon Monoxide Transfer Factor – TLCO)
- If the distance that the gas has to travel from alveolus to blood is increased (i.e. thickening of blood-gas barrier) e.g. in interstitial lung diseases where the alveolar wall is thickened, or in pulmonary oedema, the TLCO is lower than normal.
- If there is alveolar wall destruction such as in emphysema, there is less area for diffusion to occur and again TLCO will be low.
List the reactions of CO2 in blood
Carbon dioxide is an essential part of the buffer systems which controls the pH of ECF. Although CO2 is transported in venous blood, there is a substantial amount in arterial blood which has a vital role in acid base balance.
[*] There is almost three times as much CO2 in arterial blood as there is oxygen.
[*] Dissolution of Co2 in water: at a pCO2 of 5.3 kPa, water dissolves 1.2 mmol/L. Dissolved CO2 can then react with water in different components of blood.
- CO2 dissolving in plasma
- CO2 dissolving in the red blood cell
- CO2 forming carbamino compounds
Explain about Carbon Dioxide in plasma
[*] Carbon Dioxide in Plasma: CO2 dissolves in water (more soluble than O2)
- May form hydrogen and hydrogen carbonate ions
- Reaction is highly reversible
- The extent of dissociation, however, determines the pH of plasma and therefore ECF. The equilibrium is driven towards dissociation by rises in dissolved CO2. The amount of CO2 which dissolves is directly proportional to pCO2
- Dissociation is resisted by the higher concentration of HCO3- ions present in plasma (normally 25mmol/L)
- Hydrogen carbonate is not formed from CO2 in plasma but from a reaction of CO2 in red blood cells).
- pH of plasma depends on how much Co2 reacts to form H+ which depends on {dissolved Co2} pushing the reaction one way and [HCO3-} pushing it the other. Dissolved Co2 depends directly on pCO2.
- pH therefore FALLS if pCO2 rises
- pH therefore RISES if [HCO3-] rises
- pH depends on the ratio of [HCO3-} to pCO2.
- This is represented mathematically by the Henderson-Hasselbalch equation
- This buffer is working far from its pK because of the excess hydrogen carbonate. In plasma, the ratio of [HCo3-} to dissolved CO2 is 20:1 (25mmol/L: 1.2mmol/L)
- Negligible hydrogen carbonate is formed from the dissolution of CO2. In body fluids with few or no other buffer systems (e.g. plasma alone, CSF), the concentration of hydrogen carbonate is not significantly affected by changes in pCO2 and remains effectively constant over al physiological values of pCO2 unless changed by other mechanisms.
- Hydrogen carbonate in plasma (25mmol/L) is not from CO2 in plasma (sodium hydrogen carbonate) as it stops nearly all dissolved CO2 from reaction so pH is slightly alkaline.
Explain about Carbon Dioxide in the red cell
[*] Carbon Dioxide in the Red Cell: CO2 reacts with water (froms H+ and HCO3-). Reaction is reversible depending on concentration of reactants
- One of the products is removed: the hydrogen ions are bound to Hb, which has a large buffering capacity, enhanced further when it is deoxygenated.
- CO2 + H20 <=> H2CO3 , <=> H+ + HCO3-
- H+ + Hb <=> HbH
- The reaction is therefore ‘drawn’ towards the reaction of lots of CO2 and the production of HCO3-. The amount produced depends primarily upon the buffering effects of haemoglobin with only minor effects of changes in pCO2. The more H+ binds to Hb, the more Co2 reacts and the more HCO3- forms. The reactions of CO2 in the red cell are mostly determined by how much H+ binds to Hb.
- The HCO3- formed in large quantities is exporte from the red cell in exchange for inward movement of Cl-. The 25mmol/L of HCO3- in plasma is determined much more by the buffering capacity of haemoglobin than the pCO2.
- Hydrogen carbonate leaves red cell down concentration gradient in exchange for inward movement of chloride, forming the 25 mmol/L of HCO3- in plasma.
- The pH of the body fluids is therefore determined by the relationship between the amount of CO2 dissolved in plasma and the amount of HCO3- formed from CO2 in the red cell by a reaction involving haemoglobin.
- It is the ratio of HCO3- : pCO2 which matters, not absolute values
- Breathing affects arterial pCO2 and therefore the pH of body fluids
Explain about carbamino compound formation
Carbon dioxide also reacts directly with the protein part of haemoglobin, forming Carbamino compounds (carbaminohaemoglobin). This contributes to CO2 transport but does not affect acid base balance. A bit more is formed in venous blood because pCO2 is higher. If Hb is tense, it forms more carbamino compounds (physiologically significant)
Write the Henderson-Hasselbalch equation and be able to calculate the plasma pH, given the pCo2 and [HCO3-]
[*] pH = pK + log([HCO3-]/(pCo2 x 0.23))
[*] pKa = 6.1; pCO2 is in kPa and 0.23 is the CO2 solubility constant
[*] In plasma there is 20 times as much hydrogen carbonate as dissolved CO2) so log 20 = 1.3
[*] pH = 6.1 + 1.3 = 7.4
[*] In the whole body the kidney controls the hydrogen carbonate concentration in plasma by variable excretion so really pH = 6.1 + log (kidneys/lungs)
State the factors influencing the hydrogen carbonate concentration of plasma
[*] In plasma, CO2 dissolves in plasma and undergoes a slow reaction (little carbonic anhydrase) with water creating HCO3-
[*] In Red Blood Cells, CO2 also reacts with water rapidly (carbonic anhydrase is present in the RBC) to form H+ and HCO3-
[*] H+ ions bind to Hb, drawing the reaction towards HCO3- production. The amount of HCO3- produced depends primarily upon the buffering effects of Hb.
Describe the buffering action of haemoglobin in red cells
[*] H+ ions bind to Hb so it acts as buffer by mopping up H+ ions. This drives the reaction of CO2 with water in red blood cells, producing more H+ ions and HCO3-
[*] If the body produces acid, this reacts with HCO3- to form CO2 which is breathed out and stops pH changing too much. The net effect of producing acid is that it decreases the [HCO3-] in plasma and alters the pH (at least temporarily)
Describe the content of venous blood
In venous blood the pCO2 is normally 6.0 kPA. Haemoglobin is less saturated with oxygen so is a better buffer; the more relaxed Hb is, the more easily it binds O2. The more tense Hb is, the more easily it binds H+ ions cuasing more CO2 to dissolve in the red cell and therefore more HCO3- is formed which is exported to plasma. But as both pCO2 and [HCO3-} is increasing in venous blood, pH does not change much (it is the ratio that determines pH)
- 600ml of plasma contains 1.33 mmol/L dissolved CO2 i.e. 0.80mmol/L blood
- 600ml of plsma contains 27mmol/L HCO3- i.e. 16.19 mmol/L blood
- 400ml of red cells contains 0.98mmol/L dissolved CO2 i.e. 0.39 mmol/L blood
- 400ml of red cells contains 2.9mmol/L carbamino i.e. 1.17mmol/L blood
- 400ml of red cells contains 11.69 mmol/L HCO3- i.e. 4.66 mmol/L blood
TOTAL: 23.21 mmol/L
[*] When venous blood reaches the lungs, Hb picks up oxygen (becomes more relaxed) so gives up H+ which reacts with hydrogen carbonate to form CO2 which is breathed out. This property of Hb ensures all CO2 is picked up in the tissues and gives up H+ at the lungs to allow CO2 to be breathed out.
